Direct Selective Epitaxy of 2D Sb2Te3 onto Monolayer WS2 for Vertical p-n Heterojunction Photodetectors

Two-dimensional transition metal dichalcogenides (2D-TMDs) possess appropriate bandgaps and interact via van der Waals (vdW) forces between layers, effectively overcoming lattice compatibility challenges inherent in traditional heterojunctions. This property facilitates the creation of heterojunctions with customizable bandgap alignments. However, the prevailing method for creating heterojunctions with 2D-TMDs relies on the low-efficiency technique of mechanical exfoliation. Sb2Te3, recognized as a notable p-type semiconductor, emerges as a versatile component for constructing diverse vertical p-n heterostructures with 2D-TMDs. This study presents the successful large-scale deposition of 2D Sb2Te3 onto inert mica substrates, providing valuable insights into the integration of Sb2Te3 with 2D-TMDs to form heterostructures. Building upon this initial advancement, a precise epitaxial growth method for Sb2Te3 on pre-existing WS2 surfaces on SiO2/Si substrates is achieved through a two-step chemical vapor deposition process, resulting in the formation of Sb2Te3/WS2 heterojunctions. Finally, the development of 2D Sb2Te3/WS2 optoelectronic devices is accomplished, showing rapid response times, with a rise/decay time of 305 μs/503 μs, respectively.

Lasing Effect in Symmetrical van der Waals Heterostructured Metasurfaces Due to Lattice-Induced Multipole Coupling

New practical ways to reach the lasing effect in symmetrical metasurfaces have been developed and theoretically demonstrated. Our approach is based on excitation of the resonance of an octupole quasi-trapped mode (OQTM) in heterostructured symmetrical metasurfaces composed of monolithic disk-shaped van der Waals meta-atoms featured by thin photoluminescent layers and placed on a substrate. We revealed that the coincidence of the photoluminescence spectrum maximum of these layers with the wavelength of high-quality OQTM resonance leads to the lasing effect. Based on the solution of laser rate equations and direct full-wave simulation, it was shown that lasing is normally oriented to the metasurface plane and occurs from the entire area of metasurface consisting of MoS2/hBN/MoTe2 disks with line width of generated emission of only about 1.4 nm near the wavelength 1140 nm. This opens up new practical possibilities for creating surface emitting laser devices in subwavelength material systems.

Tuning and exploiting interlayer coupling in two-dimensional van der Waals heterostructures

Two-dimensional (2D) layered materials can stack into new material systems, with van der Waals (vdW) interaction between the adjacent constituent layers. This stacking process of 2D atomic layers creates a new degree of freedom-interlayer interface between two adjacent layers-that can be independently studied and tuned from the intralayer degree of freedom. In such heterostructures (HSs), the physical properties are largely determined by the vdW interaction between the individual layers,i.e.interlayer coupling, which can be effectively tuned by a number of means. In this review, we summarize and discuss a number of such approaches, including stacking order, electric field, intercalation, and pressure, with both their experimental demonstrations and theoretical predictions. A comprehensive overview of the modulation on structural, optical, electrical, and magnetic properties by these four approaches are also presented. We conclude this review by discussing several prospective research directions in 2D HSs field, including fundamental physics study, property tuning techniques, and future applications.

Time-Resolved Growth of 2D WSe2 Monolayer Crystals

Understanding and controlling the growth evolution of atomically thin monolayer two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) are vital for next-generation 2D electronics and optoelectronic devices. However, their growth kinetics are not fully observed or well understood due to the bottlenecks associated with the existing synthesis methods. This study demonstrates the time-resolved and ultrafast growth of 2D materials by a laser-based synthesis approach that enables the rapid initiation and termination of the vaporization process during crystal growth. The use of stoichiometric powder (e.g., WSe₂) minimizes the complex chemistry during the vaporization and growth process, allowing rapid initiation/termination control over the generated flux. An extensive set of experiments is performed to understand the growth evolution, achieving subsecond growth as low as 10 ms along with a 100 μm/s growth rate on a noncatalytic substrate such as Si/SiO₂. Overall, this study allows us to observe and understand the 2D crystal evolution and growth kinetics with time-resolved and subsecond time scales.

Synthesis of a Selectively Nb-Doped WS2-MoS2 Lateral Heterostructure for a High-Detectivity PN Photodiode

In this study, selective Nb doping (P-type) at the WS₂ layer in a WS₂-MoS₂ lateral heterostructure via a chemical vapor deposition (CVD) method using a solution-phase precursor containing W, Mo, and Nb atoms is proposed. The different chemical activity reactivity (MoO₃ > WO₃ > Nb₂O₅) enable the separation of the growth temperature of intrinsic MoS₂ to 700 °C (first grown inner layer) and Nb-doped WS₂ to 800 °C (second grown outer layer). By controlling the Nb/(W+Nb) molar ratio in the solution precursor, the hole carrier density in the p-type WS₂ layer is selectively controlled from approximately 1.87 × 10⁷/cm² at 1.5 at.% Nb to approximately 1.16 × 10¹³/cm² at 8.1 at.% Nb, while the electron carrier density in n-type MoS₂ shows negligible change with variation of the Nb molar ratio. As a result, the electrical behavior of the WS₂-MoS₂ heterostructure transforms from the N-N junction (0 at.% Nb) to the P-N junction (4.5 at.% Nb) and the P-N tunnel junction (8.1 at.% Nb). The band-to-band tunneling at the P-N tunnel junction (8.1 at.% Nb) is eliminated by applying negative gate bias, resulting in a maximum rectification ratio (10⁵) and a minimum channel resistance (10⁸ Ω). With this optimized photodiode (8.1 at.% Nb at V_g = -30 V), an I_photo/I_dark ratio of 6000 and a detectivity of 1.1 × 10¹⁴ Jones are achieved, which are approximately 20 and 3 times higher, respectively, than the previously reported highest values for CVD-grown transition-metal dichalcogenide P-N junctions.

First-Principles Study of Electronic and Optical Properties of Tri-Layered van der Waals Heterostructures Based on Blue Phosphorus and Zinc Oxide

The creation of van der Waals heterostructures with tunable properties from various combinations of modern 2D materials is one of the promising tasks of nanoelectronics, focused on improving the parameters of electronic nanodevices. In this paper, using ab initio methods, we theoretically predict the existence of new three-layer van der Waals zinc oxide/blue phosphorus/zinc oxide (ZnO/BlueP/ZnO) heterostructure with AAA, ABA, ABC layer packing types. It is found that AAA-, ABA-, and ABC-stacked ZnO/BlueP/ZnO heterostructures are semiconductors with a gap of about 0.7 eV. The dynamic conductivity and absorption spectra are calculated in the wavelength range of 200–2000 nm. It is revealed that the BlueP monolayer makes the greatest contribution to the formation of the profiles the dynamic conductivity and absorption coefficient spectrums of the ZnO/BlueP/ZnO heterostructure. This is indicated by the fact that, for the ZnO/BlueP/ZnO heterostructure, conductivity anisotropy is observed at different directions of wave polarization, as for blue phosphorus. It has been established that the absorption maximum of the heterostructure falls in the middle ultraviolet range, and, starting from a wavelength of 700 nm, there is a complete absence of absorption. The type of layer packing has practically no effect on the regularities in the formation of the spectra of dynamic conductivity and the absorption coefficient, which is important from the point of view of their application in optoelectronics.